Each of the nine current members of the protein arginine methyltransferase (PRMT) family has substrates whose differential methylations can lead to distinct physiological changes in the cell. However, although many in vivo substrates have been identified, many others have not. For example, the substrates of PRMT2 have not been characterized due to lack of measurable activity. Cellular roles that have been established in arginine methylation include the involvement of RNA splicing, processing and transport, transcriptional controls, DNA repair, and intracellular signaling. This proposal will set forth specific aims to characterize the newest member of the methyltransferase family, PRMT9. The first step would be to determine enzyme specificity of methylable substrates. Once the substrates are identified, and PRMT9 is characterized by its type of methyltransferase activity, I will determine its localization and its tissue expression pattern. These results will contribute to our understanding of the overall significance of methyltransferases in cellular processes.